A few generations back Her Glorious Majesty decreed the creation of the Imperial Rail Ways.
Now, you might immediately think of steam trains and steel rails. Stop that.
See, Her Majesty ruled over a large, mostly flat forest with very few rivers (but excellent groundwater thanks to a system of subsurface caves and aquifers). Moving freight from her more remote dominions to the Capital was tricky (even by road). Other kingdoms move such loads via river or canal barge, but sadly the geography in the heart of the Empire does not support doing this.
The relative abundance of wood and able carpenters meant that laying wooden rails and pulling dedicated trains of wagons with specific gauges (width between the wheels) and flanges or grooves (that keep it on the rail) across some regions was a reasonable solution (plus having a nice flat road with an easy way to pull wagons of supplies doesn't hurt troop movement).
These wooden rails have since become more and more convoluted and complex and entire shunting yards made of nothing but hardwood and polish exist in some cities. Nowadays the rails are maintained and kept clear of debris by the wagon teams and dedicated Imperial carpenters. Power for the trains used to be provided by animals walking in front of the wagon trains but (since they still needed to stop for food and rest) is now provided by Locomotive Galleys.
A Locomotive Galley is, in its simplest form, a handcar. Handcars are simple contraptions with a reciprocating arm fastened to a wheel and a lever. When the lever is pushed down/pulled up at the right time it pushes the wheel around. Needless to say Locomotive Galleys are more complex. Selectable gearing, a roof and a dedicated crew tender wagon means that an Imperial Locomotive can keep rolling day or night and deliver goods across vast swathes of the empire. They are almost entirely wood, as although metal may be used for certain key components it is hand-crafted and generally hard to replace (no industrial metalworking yet).
The question is just how large these Locomotive Galleys can be built and how long the trains they pull can be before the wood, rope, or men that comprise the train give out.
The technology for bogeys onto which the wheels can be put exists and allows for long locomotives (although I think the driving wheels must be fixed, given the constraints of carpentry). Wooden gearing exists that can be used to down or upshift the torque, though the more force is needed the heavier the wheels will become. If needed a canvas 'bike chain' can be used between two gears. A carpenter will always be present on the train to effect minor repairs and maintain the woodwork if needed, and there is a gratuitous supply of lard available to use as lubricant. No meaningful gradient is expected on the rails.
The engine can be given a starting push by any number of crew at a station but must be capable of continuing to move using only the power of the men inside, and minimal metalwork is preferable (for example an answer using only wooden axles will be preferred over one with metal ones, as long as the friction between axle and locomotive won't cause the whole thing to burst into flame). The technology level is roughly High Middle Ages (1000-1400 AD) and Western European though I'm happy to have some small innovations here and there (for example the Empire uses standardised intermodal shipping crates, a highly unusual concept for medieval times)
Added details: I’m aiming for 100 miles per day, or a sustained 5mph with some station stops. Obviously if I can’t make my trains big enough to carry their own food/replacement crews this number will have to come down, but I’m more worried about mechanical stresses under constant load than I am high accelerations. You can also assume the rails are made of a harder wood than the locomotive itself, so any failure points will be in the train rather than the rails.
Basically: How big can a train made primarily of wood, rope and sheer bloody mindedness get?